3. Write a Python program to check whether a given a binary tree is a valid binary search tree (BST) or not. Go to the editor

Let a binary search tree (BST) is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.